A membrane fuel pump of this kind is driven as a pulse pump by the changing pressure in the crankcase of the engine and is utilized, for example, with internal combustion engines of motor-driven chain saws. When an underpressure is present in the crankcase, the membrane of the fuel pump bends in the direction of a volume reduction of the drive chamber and a corresponding volume increase of the pump chamber whereby fuel is drawn by suction via the first check valve into the pump chamber. With the overpressure condition which follows, the membrane is deflected in the sense of a volume increase of the drive chamber and a corresponding volume reduction of the pump chamber whereby fuel is pumped via the second check valve on the pressure end of the pump to the membrane carburetor.
Since the pressure in the crankcase fluctuates approximately between -0.2 and 0.6 bar in dependence upon rotational speed, the feed pressure at the pressure end of the membrane fuel pump also fluctuates. For this reason, a pressure regulator is provided ahead of the entrance into the carburetor which in intended to compensate for large pressure fluctuations. From the control chamber of the pressure controller, a main nozzle channel leads into the intake pipe of the engine via a main nozzle and a check valve. An idle nozzle channel conducts fuel from the control chamber via an idle nozzle into the intake pipe.
A low underpressure must develop in the control chamber in order to hold the check valve of the main nozzle closed so that a fuel feed into the venturi section of the intake pipe is obtained exclusively via the idle nozzle channel.
In practice, it has been shown that for external accelerations acting on the control system in idle, the pressure controller cannot equalize the large pressure fluctuations of the fuel pump to the extent that the underpressure in the control chamber is obtained which is required for a reliable closure of the check valve of the main nozzle. For this reason, positive pressures in the control chamber develop also at idle whereby fuel comes out of the main nozzle (main nozzle dripping) and the mixture drawn in by suction becomes intensely overrich thereby causing the engine to die.